(1) Field of the Invention
The present invention relates to a wireless communication device, an equalizer, a computer-readable medium storing a program for obtaining weight coefficients in the equalizer, and a process for obtaining weight coefficients in the equalizer. In particular, the present invention relates to a wireless communication device and an equalizer which performs adaptive equalization with superior equalization characteristics and a small amount of calculation, and a computer-readable medium storing a program for obtaining weight coefficients in the equalizer and a process for obtaining weight coefficients in the equalizer so as to realize superior equalization characteristics and reduction in the amount of calculation.
(2) Description of the Related Art
Currently, high-speed, large-capacity mobile communication systems are being actively studied. For example, development of high-speed wireless communication systems using CDMA (Code Division Multiple Access), represented by the systems using HSDPA (High Speed Downlink Packet Access), is proceeding. Since the development of the systems using HSDPA is aimed at achievement of the transmission rate ten times or more higher than the conventional technique, implementation of an equalizer is contemplated.
In the conventional transmission systems, signal processing called equalization is performed by use of an equalizer. The equalization is processing for reducing distortion occurring during propagation through the transmission line, and normally, the equalizer provided on the receiver side can recover the distortion of the frequency characteristic.
For example, each wireless signal transmitted in mobile communications reaches a receiver after propagation through multiple paths. That is, waves of each signal propagate a plurality of paths because of reflection by mountains, buildings, and the like. Since the waves of each signal reach the receiver at different times depending on the propagation path lengths, delay distortion occurs in the waveform of the received signal. In addition, since reflection and scattering are repeated before the reach to the receiver, amplitude distortion also occurs.
When delay distortion or amplitude distortion occurs, intersymbol interference (in which successively transmitted pulses overlap) occurs, so that the pulses cannot be correctly discriminated by the receiver. Therefore, in order to remove the intersymbol interference and compensate for deterioration of the transmission quality, filters called equalizers are used. The equalizers include, for example, adaptive equalizers, which equalize signals according to change in the characteristics of a transmission line caused by movement of a mobile station and the like.
Specifically, the adaptive equalizers are learning filters, each of which refers to a desired signal (which is called a training signal, and is a signal to be transmitted through a certain transmission line and recovered), and adjusts weight coefficients (tap coefficients) in the filter so that the output of the filter approaches the desired signal. In particular, the transversal equalizer (which uses a transversal filter) is a kind of the adaptive equalizer, and can cope with high-speed fading.
However, for the above equalizers, solving of linear equations expressed by a correlation matrix in and/or between propagation channels is required for obtaining synthesized weights called weight coefficients (tap coefficients) on the basis of the MMSE (Minimum Mean Square Error) criterion. (See, for example, Japanese Unexamined Patent Publication No. 2006-311083.)
Generally, the characteristics of the equalizer are improved with increase in the number of the timings of synthesis (i.e., the number of weight coefficients). Nevertheless, the amount of calculation necessary for solving the linear equations increases in proportion to the cube of the number of the timings of synthesis. Therefore, the performance of the equalizer is limited by the trade-off between the maximum number of the timings of synthesis and the amount of calculation.